1. Field of the Invention
This invention relates generally to photodetectors and more particularly to room temperature sensors for imaging ultraviolet radiation using III-V compound semiconductor photodetectors.
2. Description of the Related Art
Ultraviolet sensors transform incident light in the ultraviolet region of the spectrum (wavelengths from approximately 0.01 to 0.40 microns) into electrical signals that are used for data collection, processing, and storage. Such sensors can capture images for video or still-frame imaging. Conventional solid-state video sensors usually employ silicon photodetectors because they are inexpensive, exhibit adequate signal bandwidth, are sensitive to visible and near-infrared radiation, and exhibit a high degree of pixel-to-pixel uniformity when used in an imaging array.
Silicon photodetectors exhibit significant deficiencies when applied at room temperature for ultraviolet wavelength detection, however. At or near room temperature, these detectors have a characteristic xe2x80x9cdark currentxe2x80x9d which is very large compared to the signal and causes prominent noise characteristics. The dark current is a fundamental consequence of the detector physics: common silicon photodetectors exhibit an energy bandgap of 1.12 eV, which is well below the threshold for ultraviolet wavelength absorption. This low bandgap gives rise to a large dark current (at normal room temperatures). Silicon photodetectors that are compatible with low to moderate cost production also have near unity gain: i.e. each incident photon generates at most a single electron. The combination of low gain and large dark currents limits the practical application of Silicon photodiode detectors to conditions having relatively bright irradiance, unless active cooling is used. At low light levels and at room temperature such detectors generate inadequate signal-to-noise ratios (SNR).
Although the practical short wavelength limit for silicon is approximately 250 nm, it can be used at wavelengths as short as 190 nm. UV radiation, however, readily damages silicon detectors. Degradation in responsivity occurs after only a few hours of ultraviolet exposure and makes the devices unusable for precision measurements.
In low-light-level conditions like those encountered in detecting ultraviolet radiation, the conventional silicon photodiode detector is often replaced with a silicon avalanche detector to facilitate gain within the detector so that conventional detector interface amplifiers and ancillary interface circuits can be used to read out the data at video frame rates with a high SNR. Such applications of avalanche photodiodes are disclosed by U.S. Pat. Nos. 5,146,296 to Huth and 5,818,052 to Elabd, for example. Unfortunately, the fabrication of avalanche photodiodes is much more difficult and expensive than standard photodiodes, and supplemental amplification is also often required. Currently available avalanche photodiodes exhibit relatively poor uniformity and have limited sensitivity due to their low quantum efficiency. They are also inherently non-linear in their response to light, which is undesirable in many applications.
Alternative ultraviolet imaging systems are known which use an array of avalanche detectors, various phosphors, or intensifiers such as microchannel plates to amplify the available electrons for subsequent detection in enhanced charge coupled devices (CCDs) . All such CCDs and other metal-insulator-semiconductor (MIS) devices have surface states at the semiconductor/insulator interface that cause spontaneous generation of dark current. Furthermore, the soft x-rays associated with electron bombardment damage intensified CCDs. This damage manifests as even higher dark current that reduces dynamic range, both by consuming charge-handling capacity and by adding noise. CCD Manufacturers employ various schemes to suppress dark current, such as that described by Saks, xe2x80x9cA Technique for suppressing Dark Current Generated by Interface States in Buried Channel CCD Imagers,xe2x80x9d IEEE Electron Device Letters, Vol. EDL-1, No. 7, Jul. 1980, pp. 131-133. Nevertheless, mid-gap states are always present that result in unacceptable dark current for room temperature operation of silicon-based low-light-level image sensors.
A further problem with prior detectors arises from their spectral response characteristics. The large mismatch between the photoresponse required for detecting ultraviolet radiation and the actual spectral response of silicon photodetectors results in higher dark current because the bandgap is much lower than necessary. In semiconductor detectors the semiconductor bandgap determines the long wavelength detection limit. At wavelengths longer than the bandgap the material becomes transparent. The depletion layer must be made very thin to absorb radiation at very short wavelengths.
In summary, conventional ultraviolet photodetectors are subject to a variety of practical limitations: CCDs, both conventional and intensified, have inadequate sensitivity in the ultraviolet part of the electromagnetic spectrum because their dark current is too high at room temperature and their design is not optimized for detecting ultraviolet radiation. Pyroelectric detectors, which respond only to pulsed radiation, and thermal detectors have inadequate time constants for broad use. Photomultiplier tubes are fragile. Optical downconversion techniques are susceptible to physical damage in the sensitive crystal probe used to produce visible fluorescence for subsequent detection in a CCD sensor.
In view of the above problems, the present invention is a high-sensitivity photodetector for detecting radiation in the ultraviolet region of the electromagnetic spectrum, suitable for operation at room temperature. The photodetector includes (a) a compound semiconductor photodiode which generates a detector current in response to incident photons, the photodiode biased below its avalanche breakdown threshold, comprising III-V elemental components and having a bandgap with transition energy higher than the energy of visible photons; and (b) MOS detector interface circuit at each pixel, arranged to receive a signal from the photodiode junction and to amplify said signal.
Preferably, the photodetector has its photodiode junction integrated in a first microstructure on a first substrate, and its interface circuit in a second microstructure on a second substrate. The first and second microstructures are then joined in a laminar, sandwich-like structure. The first and second microstructures communicate via electrically conducting contacts.
In one embodiment, the photodetectors are integrated in an imaging array for use at room temperature to detect individual photons and generate video at TV-compatible and higher frame rates. Such an imaging array is made up of a plurality of addressable photodetecting cells. Each cell includes a compound semiconductor photodiode which linearly generates a detector current in response to incident photons, the photodiode biased below its avalanche breakdown threshold, comprising III-V elemental components and having a bandgap with transition energy higher than the energy of visible photons. Each cell also includes a MOS detector interface circuit at each pixel, arranged to receive a signal from the photodiode junction and amplify the signal.
In some embodiments, the interface circuits of at least some cells have independently variable gain. The gain at each pixel can thus be set to compensate for non-uniform photodiode response across the array.
The photodetector and array of the invention provide single-photon sensitivities, with higher signal-to-noise ratios at video frame rates and room temperature than previously possible, for detecting and imaging radiation in the ultraviolet region of the spectrum.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: